The present invention relates generally to methods, apparatus, computer-readable media and processors for performing network handovers in mobile telecommunication systems, and more particularly, to methods, devices and computer program products for performing inter Radio Access Technology (RAT) handovers from a Universal Mobile Telecommunications Service (UMTS) system to a Global System for Mobile communications (GSM) system.
GSM (Global System for Mobile communication) is a digital mobile phone system utilized in Europe and other areas. GSM is a variation of TDMA (Time Division Multiple Access), which along with CDMA (Code Division Multiple Access), are the three major digital radio (wireless) phone technologies currently being used the most. GSM digitizes and compresses data, then sends it down a channel with two other streams of user data, each in its own time slot.
UMTS (Universal Mobile Telecommunications Service) is a third-generation (3G) broadband, packet-based transmission of text, digitized voice, video, and multimedia at data rates up to 2 megabits per second. Based on the GSM communication standard, UMTS is at present being employed in various geographic regions. Once UMTS is fully available geographically, users will have access through a combination of terrestrial wireless and satellite transmissions. Until UMTS is fully implemented, however, users may utilize multi-mode devices that switch to the currently available technology (such as GSM 900 and 1800) where UMTS is not yet available.
A problem arises because the technical transition from the second generation cellular system (i.e. GSM/GPRS) to the third generation mobile telephony system (i.e. UMTS) will take place step by step, probably over a number of years. As such, for the time being, many network operators feel the necessity of providing users with mobile terminals that work in both GSM/GPRS and UMTS modes. As such, a dual mode user equipment or dual mode mobile terminal that supports UMTS as well as GSM/GPRS was developed. Further, since many mobile terminal users travel from one region to another region, where different radio access technologies are covered, there is a necessity for a multi-mode mobile terminal. Thus, even though UMTS is not yet completely developed, mobile subscribers can be provided with multiple services by using dual mode mobile terminals that supports both UMTS and GSM/GPRS whenever necessary. Consequently, when they move to a region where there is no coverage provided for UMTS users, the call still can be set up on an existing network, e.g. GSM 900 or GSM 1800.
Additionally, dual-mode mobile terminals provide for a user to switch between UMTS and GSM during a call. Such switching may be required if the UMTS service is not available in a region, or if the signal is degraded such that service is inadequate. Such switching of network services during the call mode is referred to as an inter-Radio Access Technology handover, i.e., switching a channel to a new RAT network while maintaining the existing GSM/GPRS or UMTS connection.
Up until now, the decision process that a mobile terminal uses for determining whether to handover over a call from the UMTS service to the GSM service has been based on one or more monitored radio measurements meeting “hard” limits or “hard” thresholds. The term “hard” limit or threshold is utilized because the measured value of the monitored radio measurement will either trigger the handover or it will not trigger the handover. For example, during a call, the mobile terminal may monitor the state of the existing UMTS service as well as the state of a GSM service which may be available for a handover. Specifically, for the UMTS service, the mobile terminal may monitor radio measurements such as signal strength in the form of Received Signal Code Power (RSCP) and/or interference in the form of signal-to-interference ratio (Ec/No). For the GSM service, the mobile terminal may monitor the signal strength in the form of Received Signal Strength Indication (RSSI).
Currently, a single threshold is set for each monitored radio measurement and the decision on whether to handover the call to the GSM service is based on whether or not one or some combination of the thresholds are met. For example, in current handover triggering decision implementations, if one or any combination (depending on the implementation) of the UMTS measurements fall below their respective threshold, which indicates that the UMTS service has degraded below prescribed limits, and if the GSM measurement is above the respective threshold, which indicates that the GSM service is currently at an acceptable signal strength level, then triggering of a handover occurs. If one or both of the UMTS measurements are above their respective thresholds or if the GSM measurement is below the respective threshold, however, then the mobile terminal determines that the current state of the service is adequate and a handover is not triggered.
This so-called hard decision-making provides only limited flexibility in the decision process. For example, the UMTS measurements may be very close to falling below the threshold, while the GSM measurement may be well above the threshold, which would indicate strong GSM signal strength. However, in this example, under the current decision scheme, the mobile terminal would remain with the UMTS service because the UMTS service quality has not fallen below the hard threshold limits. Similarly, for example, the UMTS measurements may fall well below their respective thresholds, indicating poor UMTS service quality, while the GSM measurement is marginally below the threshold. In this example, under the current decision making scheme, the mobile terminal would remain with the UTMTS service because the GSM service is slightly below the respective threshold. The inflexibility of these hard-decision schemes is evident in that handover does not occur even though the UMTS service has degraded to an unacceptable level because the GSM service marginally fails to meet a level of acceptance, or in that the UMTS service is marginally above requisite quality levels and thus no handover occurs even though the GSM network is significantly above the level of quality required to perform a handover. This type of “go”/“no go” decision-making does not adequately address the utilization of network resources to insure optimization.
Therefore, a need exists to develop a handover decision-making process that relies on one or more soft-decisions, otherwise referred to as fuzzy logic. Soft-decisions would allow for all handover measurements to be considered relative to one another in making a handover decision. A soft-decision making process would result in a handover triggering scheme that is more flexible in terms of network management of the decision process and provides for better optimization of the use of network resources.